The present invention relates generally to half bridge resonant-type power converters. More particularly, the present invention relates to LED drivers including circuitry for pre-charging one or more energy storage elements in the resonant tank to reduce turn-on current spikes.
Half-bridge resonant type converters are currently very popular, at least because of their relatively wide output range and high efficiency. A first typical topology for a half-bridge resonant type converter 10a is illustrated by reference to FIG. 1. Switching elements Q1 and Q2 are connected in series to form a half-bridge with an output node 16 there between. A resonant tank is coupled to the output node 16 and in the present example is formed by an inductor L1 and a capacitor C2 as the primary resonant components for the resonant tank, as well as a load 14 and a DC blocking capacitor C3 to substantially prevent DC current from going through the resonant inductor L1 and the load 14. A controller 12 is configured to provide gate drive signals to drive the half bridge switching elements Q1 and Q2. The half bridge switching elements Q1 and Q2 are coupled across a DC voltage source V1, further in parallel with a resistor R4 and capacitor C1 that serve as a power supply circuit for the controller. Body diodes D1 and D2 are provided for switching elements Q1 and Q2 that help with free-wheeling the negative current. By adjusting the gate drive frequency the output can be adjusted easily.
In an alternative conventional topology as illustrated in FIG. 2, the DC blocking capacitor C3 can be connected in the circuit in a different way and still have the same function. Here, the DC blocking capacitor C3 is coupled between the half bridge output node 16 and the resonant inductor L1.
When well designed, the resonant tank (e.g., formed by the resonant inductor L1, resonant capacitor C2, DC blocking capacitor C3, and the load 14) may be inductive in nature. In other words, the inductor current I_L1 will always be lagging the voltage at the half bridge output node 16 such that the half-bridge switching elements will maintain soft-switching operation, and thereby reduce switching losses with relatively high efficiency.
Referring now to FIG. 3, steady state switching operation and associated waveforms are provided for further illustration. As shown therein, steady state gate drive signals G1 and G2 (for Q1 and Q2, respectively) are quasi-square wave voltage signals with duty ratios of slightly less than 50%. The resonant inductor current I_L1 is shown to be lagging the half bridge output voltage V_16. After gate drive signal G2 turns off the low-side switching element Q2, gate drive signal G1 turns high to turn on the high-side switching element Q1. The inductor current I_L1 is negative before the high-side switching element Q1 turns on. This negative current will be forced into the body diode of the high-side switching element Q1 and force the voltage across this switching element Q1 to 0. When the inductor current changes to positive, it will flow through the drain channel of the high-side switching element Q1. Since the high-side switching element Q1 turns on with 0 voltage across it, soft-switching (i.e., zero-voltage-turn-on) is achieved. One of skill in the art may appreciate that soft-switching helps reduce switching losses and improve converter efficiency.
FIG. 3 further illustrates the voltage V_C3 across the DC blocking capacitor C3 during stead state operation. The voltage V_C3 has a DC value of one half the positive rail voltage from the DC source (i.e., V1/2) along with a relatively small AC ripple component.
The most stressful moment for the conventional half-bridge resonant type converter is during start-up procedures. High current spikes are possible during startup because there is no energy stored in the resonant tank (e.g., L1, C2, C3).
Referring next to FIG. 4, an exemplary startup process for the controller is illustrated. The controller 12 needs input from the power supply to start up, and it typically has a turn-on threshold voltage, V_on. An input-side capacitor C1 may provide the energy for the controller 12 to start and run. A resistor R4 is coupled in series with the capacitor C1 to provide the charging path for the capacitor C1 from the rail voltage V1. When the voltage across the input capacitor C1 reaches the turn-on threshold voltage V_on, the controller begins generating gate drive pulses G1 and G2. The controller generates the low side switch gate drive signal G2 first to allow the boot-strap circuit to be charged up for the high side switch gate drive.
A typical switch current waveform during startup of the half bridge circuit is shown in FIG. 5. Before the half bridge switching elements Q1 and Q2 are turned on, there is no energy stored in the resonant tank. Accordingly, when the high side switch Q1 turns on the DC power supply V1 will force a series of large current pulses to charge up the resonant tank. The current spike can often approach or even exceed 10 A, and is unpredictable as well. The large current spike puts a significant amount of stress on the half bridge switching elements and can cause damage to these and other power converter components.